Previously, I mentioned that one of the consequences of a wizard-scarce fantasy world is that there will be a dangerously short supply of magic weapons with which to dispatch those foes who are unaffected by mundane weapons. A scarcity of magic weapons is, I should mention, a desirable feature, not a flaw. The scarcer they are, the greater they are valued, and the more they add to the fantastical atmosphere of the adventures. Making these supernatural monsters vulnerable to non-magical weapons is not a solution. It reduces the value of the coveted magic weapon, and it reduces the impressiveness of the monster. Rather, I think the answer lies in spells. AD&D already gives us the fourth level magic-user spell enchanted weapon, but this spell is too weak for its assigned level, and it comes far too late in a magic-user's career to be useful, for by the time he or she has reached the minimum level to cast it (seventh), the adventuring party will surely have found at least one magic weapon. In fact, they probably would not have survived long enough to reach that level if they hadn't found one considerably earlier. The answer is to make the spell accessible at an earlier level. The following variant is a second level spell, accessible to magic-users and elves at the third level of experience.
This spell conforms to the wisdom of Basic/Expert D&D and Labyrinth Lord in that creatures who are only affected by magic weapons are affected by any magic weapon regardless of its bonus.
Spell Class: Magic-user
Spell Level: 2
Duration: 6 rounds per level of the caster
This spell temporarily enchants one ordinary weapon (or up to three small weapons, such as arrows, bolts, or daggers), thereby rendering it effective against foes who are invulnerable to non-magical (or non-silver) weapons. This spell does not confer combat bonuses.